IS2 and IS3 elements mediate some of the integration of F into the E. coli chromosome to produce Hfr strains. The F episomal system provides an example of how IS elements are involved in the formation of plasmid cointegrates and can be used to study recombination between IS elements. Relative recombinational activities of IS2 and IS3 will be assessed in two ways: by determining which of these two elements was involved in the formation of a number of Hfr's, and by determining which elements recombine during F. excision. To do this, F, F-prime, and Hfr DNA will be examined by using restriction enzyme analysis, Southern hybridization techniques, and electron microscope heteroduplex methodology. Excisional recombination will be examined both in recA ion and recA minus genetic backgrounds to determine how this process is related to the illegitimate recombinational processes characteristic of IS elements. Because many Hfr sites correspond to IS elements in the bacterial chromosome, F will be used as a probe to identify and map IS's within the bacterial DNA. IS classes other than IS2 and IS3 will also be sought.